The invention relates to a code-modulated transmission method and a transmission system.
A transmission system operating according to the code-modulated transmission method (CDMA) (Code Division Multiple Access) constitutes a telecommunications system in which a radio field is used jointly by a number of subscribers. The telecommunications system comprises fixed or mobile subscriber stations which are connected to a base station. In this context, the subscribers can communicate with one another via the base station. For this purpose, transmission channels are provided which, in the case of the CDMA method, as already described in German reference DE 4 333 396 A1, can be distinguished from one another by means of channel-specific encoding of the useful information to be respectively transmitted in these telecommunications channels. The channel-specific encoding takes place here at the transmit end, for example in the case of a transmission from the base station to the subscriber stations (also designated below as xe2x80x9cdownlinkxe2x80x9d transmission), in such a way that each useful information bit to be transmitted in a specific transmission channel is itself divided into a fixed number of bits designated as chips (xe2x80x9csub bitsxe2x80x9d). The chip sequence formed by this number of chips constitutes here a chip sequence which is characteristic of the respective transmission channel. Using the codes formed in this way for the individual transmission channels, a radio-frequency carrier signal is then modulated so that the individual codes are superimposed in this modulated carrier signal.
At the receive end, i.e. in the individual subscriber stations in the assumed example, the transmission channel, and thus code, which is appropriate for the respective subscriber station is initially selected from the received radio-frequency carrier signal by demodulation and filtering, and finally the useful information transmitted in the said transmission channel is recovered from it.
In the case of the CDMA method, the bandwidth of the useful information to be transmitted is to a greater extent widened as a result of the previously described encoding of said information. In this context, mention is also made of a xe2x80x9cspread spectrumxe2x80x9d transmission method. Here, the entire frequency band available is exploited for the transmission of the useful information.
In the case of a modified CDMA method, namely the TCDMA method, which constitutes a combination of the CDMA method and the known TDMA method xe2x80x9cTime Division Multiple Accessxe2x80x9d), a fixed number of time slots is fixed for each code, and thus transmission channel, so that, in time division multiplex mode, each code can be utilized by a plurality of subscribers.
European reference EP-A-430587 also discloses a transmission system working according to a code-modulated transmission method, whereby the transmission channels are represented by a plurality of radio-frequency carrier oscillations modulated by codes and the phases of the radio-frequency carrier oscillations modulated by codes are synchronous.
The object of the present invention is to provide a code-modulated transmission method in order to be able to detect, at the receive end, fixed transmission channels with specific codes in relation to other codes/transmission slots with a higher degree of reliability.
In general terms the present invention is a code-modulated transmission method for transmitting information between a base station and a multiplicity of subscriber stations in transmission channels that are represented by radio-frequency carrier oscillations differently modulated by codes. At least one of the transmission channels is emphasized, as a prioritized transmission channel, in comparison with the other transmission channels in that the phase of the radio-frequency carrier oscillations modulated by the code of the prioritized transmission channels is shifted by a fixed amount compared to the phase of the radio-frequency carrier oscillations modulated by the codes of the other transmission channels.
Advantageous developments of the present invention are as follows.
The phase shift equals 90xc2x0.
In the respective transmission channel represented by a code a multiplicity of time slots is fixed for a time division multiplex transmission of different information.
A synchronization channel which is provided for the transmission of synchronization information is selected as prioritized transmission channel.
The code which represents the prioritized transmission channel is transmitted with a higher level of energy in comparison with the codes which represent the other transmission channels.
The difference between the energy of the code which represents the prioritized transmission channel and the energy of the codes which represent the other transmission channels is 1 dB.
The present invention is also a transmission system operating according to a code-modulated transmission method for transmitting information between a base station and a multiplicity of subscriber stations in transmission channels that are represented by radio-frequency carrier oscillations differently modulated by codes. The base station is constructed such that at least one of the transmission channels is emphasized by the station, as a prioritized transmission channel, in comparison with the other transmission channels, in that the phase of the radio-frequency carrier oscillations modulated by the code of the prioritized transmission channels is shifted by a fixed amount compared to the phase of the radio-frequency carrier oscillations modulated by the codes of the other transmission channels. The subscriber stations each have detection means which are constructed such that they detect initially at least some of the codes in accordance with the phase angle, and the information transmitted is then recovered from the respective code.
The invention provides here the advantage that the respective prioritized code, and thus transmission channel, can be detected in relation to the other codes in a subscriber station with a relatively low additional control expenditure.